Implement couriers

This patch introduces the couriers, which the queue uses to deliver mail.

We have a local courier (using procmail), a remote courier (uses SMTP), and a
router courier that decides which of the two to use based on a list of local
domains.

There are still a few things pending, but they all have their basic
functionality working and tested.

internal/courier/smtp_test.go

@@ -0,0 +1,144 @@
+package courier
+
+import (
+	"bufio"
+	"net"
+	"net/textproto"
+	"testing"
+	"time"
+)
+
+// Fake server, to test SMTP out.
+func fakeServer(t *testing.T, responses map[string]string) string {
+	l, err := net.Listen("tcp", "localhost:0")
+	if err != nil {
+		t.Fatalf("fake server listen: %v", err)
+	}
+
+	go func() {
+		defer l.Close()
+
+		c, err := l.Accept()
+		if err != nil {
+			t.Fatalf("fake server accept: %v", err)
+		}
+		defer c.Close()
+
+		t.Logf("fakeServer got connection")
+
+		r := textproto.NewReader(bufio.NewReader(c))
+		c.Write([]byte(responses["_welcome"]))
+		for {
+			line, err := r.ReadLine()
+			if err != nil {
+				t.Logf("fakeServer exiting: %v\n", err)
+				return
+			}
+
+			t.Logf("fakeServer read: %q\n", line)
+			c.Write([]byte(responses[line]))
+
+			if line == "DATA" {
+				_, err = r.ReadDotBytes()
+				if err != nil {
+					t.Logf("fakeServer exiting: %v\n", err)
+					return
+				}
+				c.Write([]byte(responses["_DATA"]))
+			}
+		}
+	}()
+
+	return l.Addr().String()
+}
+
+func TestSMTP(t *testing.T) {
+	// Shorten the total timeout, so the test fails quickly if the protocol
+	// gets stuck.
+	smtpTotalTimeout = 5 * time.Second
+
+	responses := map[string]string{
+		"_welcome":          "220 welcome\n",
+		"EHLO localhost":    "250 ehlo ok\n",
+		"MAIL FROM:<me@me>": "250 mail ok\n",
+		"RCPT TO:<to@to>":   "250 rcpt ok\n",
+		"DATA":              "354 send data\n",
+		"_DATA":             "250 data ok\n",
+		"QUIT":              "250 quit ok\n",
+	}
+	addr := fakeServer(t, responses)
+	host, port, _ := net.SplitHostPort(addr)
+
+	fakeMX["to"] = host
+	smtpPort = port
+
+	s := &SMTP{}
+	err := s.Deliver("me@me", "to@to", []byte("data"))
+	if err != nil {
+		t.Errorf("deliver failed: %v", err)
+	}
+}
+
+func TestSMTPErrors(t *testing.T) {
+	// Shorten the total timeout, so the test fails quickly if the protocol
+	// gets stuck.
+	smtpTotalTimeout = 1 * time.Second
+
+	responses := []map[string]string{
+		// First test: hang response, should fail due to timeout.
+		map[string]string{
+			"_welcome": "220 no newline",
+		},
+
+		// MAIL FROM not allowed.
+		map[string]string{
+			"_welcome":          "220 mail from not allowed\n",
+			"EHLO localhost":    "250 ehlo ok\n",
+			"MAIL FROM:<me@me>": "501 mail error\n",
+		},
+
+		// RCPT TO not allowed.
+		map[string]string{
+			"_welcome":          "220 rcpt to not allowed\n",
+			"EHLO localhost":    "250 ehlo ok\n",
+			"MAIL FROM:<me@me>": "250 mail ok\n",
+			"RCPT TO:<to@to>":   "501 rcpt error\n",
+		},
+
+		// DATA error.
+		map[string]string{
+			"_welcome":          "220 data error\n",
+			"EHLO localhost":    "250 ehlo ok\n",
+			"MAIL FROM:<me@me>": "250 mail ok\n",
+			"RCPT TO:<to@to>":   "250 rcpt ok\n",
+			"DATA":              "554 data error\n",
+		},
+
+		// DATA response error.
+		map[string]string{
+			"_welcome":          "220 data response error\n",
+			"EHLO localhost":    "250 ehlo ok\n",
+			"MAIL FROM:<me@me>": "250 mail ok\n",
+			"RCPT TO:<to@to>":   "250 rcpt ok\n",
+			"DATA":              "354 send data\n",
+			"_DATA":             "551 data response error\n",
+		},
+	}
+
+	for _, rs := range responses {
+		addr := fakeServer(t, rs)
+		host, port, _ := net.SplitHostPort(addr)
+
+		fakeMX["to"] = host
+		smtpPort = port
+
+		s := &SMTP{}
+		err := s.Deliver("me@me", "to@to", []byte("data"))
+		if err == nil {
+			t.Errorf("deliver not failed in case %q: %v", rs["_welcome"], err)
+		}
+		t.Logf("failed as expected: %v", err)
+	}
+}
+
+// TODO: Test STARTTLS negotiation.